1. Field of the Invention
The present invention relates to a signal detection method in a receiver of a TDMA radio system, in which a primary signal and at least one interfering co-channel signal are received on the same TDMA channel, said signals having different but known training sequences and having different multipath propagation and thus different transmission channels on the radio path.
2. Description of Related Art
In future development of cellular mobile networks, the availability of the radio spectrum will be one of the key problems. In narrowband mobile systems, co-channel interference (CCI) is one of the main factors limiting system capacity. Conventionally, system capacity has been increased in this respect by increasing the geographical re-use of frequencies while decreasing cell size and transmission power. Of course, this is not the preferred manner, since the infrastructure may become intolerably expensive. Another possibility of solving the capacity problem is to follow the rapid development of digital signal processing and to utilize the rather complex interference cancelling algorithms. The removal of co-channel interference in a receiver enables a more efficient re-use of frequencies in a network.
In current mobile systems, co-channel signals are approximated as random additive white Gaussian noise in receivers. However, this approximation is sufficient only if interfering co-channel signals are sufficiently weak, which can be ensured by an appropriate degree of re-use of frequencies in a conventional cellular system. However, this is not the case in interference limited cellular systems. Actually, co-channel interference is typically deterministic in nature, which means that it should be possible to remove at least part of its influence.
Interference cancellation (IC) is already a popular issue in connection with code division multiple access systems (CDMA). Applying interference cancellation techniques to narrowband TDMA systems is, however, basically a more difficult task than in CDMA systems, where information on transmitted wave forms is available in advance in an interference cancelling receiver.
A radio transmitter signal usually undergoes so called multipath propagation, where the signal propagates along several different paths due to obstacles and reflections and arrives at a receiver as several signal components delayed in different ways. In digital systems, this so called time dispersion causes intersymbol interference (ISI), in which interference successive symbols are partly superimposed, which complicates demodulation in the receiver. In the pan-European mobile communications system GSM, a pre-known training sequence is included in a signal to be transmitted. This makes it is possible to estimate in a receiver the multipath channel via which the signal has travelled on the radio path. This estimated channel model allows the reciever to correct the received signal and to use the estimated channel model in the equalizer and demodulator of the receiver in the Viterbi algorithm. The GSM system specifies eight different training sequences in such a manner that it is possible to allocate different training sequences to channels using the same frequencies in cells which are sufficiently close to interfere with one another. A receiver can thus distinguish the correct signal from an interfering signal arriving at the same time. Since in a typical cellular system, such as the GSM, co-channel interference is kept, if possible, as low as possible by means of cellular network frequency planning, it is treated in a receiver merely as random Gaussian noise. Actual channel estimation, equalization and detection are performed without information on other co-channel signals. This does not cause problems in a correctly planned cellular network, where the distance between cells using the same channel is sufficiently long. However, if the utilization of a frequency band is to be increased by increasing the geographical re-use of frequencies in a cellular network, i.e. by bringing the cells using the same channel closer to one another, or by starting to use the same channels in the entire network like in CDMA systems, problems will arise. The signals superimposed in a receiver thus effectively bias the channel models estimated for each signal, which in turn degrades the performance of Viterbi decoders, for instance. For instance, in the GSM system, the cross correlation of training sequences is relatively low, which leads to biased channel estimates for co-channel users. In the GSM system, the training sequences were selected with the emphasis on the auto correlation properties thereof, since the principle in planning the system was not the optimal utilization of bandwidth, the effect of co-channel training sequences being cancelled by using an intercellular distance of at least seven cells in frequency re-use. Thus, in the current TDMA systems, increasing the degree of re-use of frequencies or allowing co-channel users is not possible without essentially degrading the performance of the system and especially that of receivers.
In "Joint Estimation Algorithms for Cochannel Signal Demodulation", K. Giridhar et al., Proceedings of ICC, 1993, and "Joint Demodulation of Cochannel Signals Using MLSE and MAPDS Algorithms, K. Giridhar et al., Proceedings of ICASSI'93, 1993, are disclosed methods in which a detection algorithm detects co-channel signals, but the actual estimates of the co-channel signals are not determined. This is thus only a partial solution to the problem.